Lecture 5 Nitrogen metabolism.docx

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Biological Sciences
Ken Scott

Lecture 5 Nitrogen metabolism, proteins as macronutrients Sunday, 23 September 2012 12:12 a.m. Nitrogen metabolism:  Purines and pyrimidine's have no dietary requirement, the human body synthesises them itself by use of salvation pathways.  Excess dietary nucleic acids my cause gout eg insoluble uric acid. Amino Acids:  20 Commonly found in proteins  Humans can synthesise about half  Remainder must be present in diet  Surplus dietary protein provide amino groups for biosynthesis, surplus carbon skeletons oxidised or otherwise metabolised.  Surplus ammonia detoxified to UREA. Protein digestion:  Acid denaturation, hydrolysis and pepsin digestion in stomach. Enzymes present in stomach are inactive precursors in pancreatic secretions.  Enterokinase on membrane of enterocytes activates trypsinogen  In small intestine:  Trypsinogen --> trypsin  Chyotrypsinogen --> chymotrypsin  Proelastase --> elastase  Peptide further degraded by CAP, CPB  Amino acids and very small peptides absorbed by intestinal cells (brush border)  Peptidases break down peptides to amino acids  Active transport from the intestinal lumen into the epithelial cells  Facilitated diffusion into blood from epithelium  Amino acid taken up by cells for protein synthesis or metabolism  A number of protein degradative pathways exist in cells but all involve proteinases.  Cellular cytoplasm proteinases are catalysed by calpains or proteasomes  Lysosomal proteinases are called cathepsins, recycle amino acids Digestibility of protein:  Denaturation by heat (cooking) improved digestibility  Globular protein are digested and absorbed completely as well most fibrous proteins with the exception of a few  Negligible residual nitrogen in faeces  Some foods rich in proteinases inhibitors which may impair digestion Turnover of body protein:  1-3% body protein degraded daily, this yields about 300g of amino acids from turnover of endogenous amino acids  Another 100g is obtained daily from a standard westernised diet  400g
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